The multiple hearth apparatus and process of the present invention is broadly applicable for the processing of organic carbonaceous materials containing residual moisture under controlled pressure and elevated temperatures to effect a desired physical and/or chemical modification thereof to produce a reaction product suitable for use as a fuel. More particularly, the present invention is directed to an apparatus and process by which carbonaceous materials containing appreciable quantities of moisture in the raw feed state are subjected to elevated temperature and pressure conditions whereby a substantial reduction in the residual moisture content of the solid reaction product is effected in addition to a desired thermal chemical restructuring of the organic material to impart improved physical properties thereto including an increased heating value on a dry moisture-free basis.
Shortages and increasing costs of conventional energy sources including petroleum and natural gas have occasioned investigations of alternative energy sources which are in plentiful supply such as lignitic-type coals, sub-bituminous coals, cellulosic materials such as peat, waste cullulosic materials such as sawdust, bark, wood scrap, branches and chips derived from lumbering and sawmill operations, various agricultural waste materials such as cotton plant stalks, nut shells, corn husks or the like and municipal solid waste pulp. Such alternative materials, unfortunately, in their naturally occurring state are deficient for a number of reasons for use directly as high energy fuels. Because of this, a variety of processes have heretofore been proposed for converting such materials into a form more suitable for use as a fuel by increasing their heating value on a moisture-free basis while at the same time increasing their stability to weathering, shipment and storage.
Typical of such prior art apparatuses and processes are those as described in U.S. Pat. No. 4,052,168 by which lignitic-type coals are chemically restructured by a controlled thermal treatment providing an upgraded solid carbonaceous product which is stable and resistant to weathering as well as being of increased heating value approaching that of bituminous coal; U.S. Pat. No. 4,127,391 in which waste bituminous fines derived from conventional coal washing and cleaning operations is thermally treated to provide solid agglomerated coke-like products suitable for direct use as a solid fuel; and U.S. Pat. No. 4,129,420 in which naturally occurring cellulosic materials such as peat as well as waste cellulosic materials are upgraded by a controlled thermal restructuring process to provide solid carbonaceous or coke-like products suitable for use as a solid fuel or in admixture with other conventional fuels such as fuel oil slurries. An apparatus and process for effecting an upgrading of such carbonaceous feed materials of the types described in the aforementioned United States patents is disclosed in U.S. Pat. No. 4,126,519 by which a liquid slurry of the feed material is introduced into an inclined reactor chamber and is progressively heated to form a substantially dry solid reaction product of enhanced heating value. The reaction is performed under a controlled elevated pressure and temperature in further consideration of the residence time to attain the desired thermal treatment which may include the vaporization of substantially all of the moisture in the feed material as well as at least a portion of the volatile organic constituents while simultaneously undergoing a controlled partial chemical restructuring or pyrolysis thereof. The reaction is carried out in a nonoxidizing environment and the solid reaction product is subsequently cooled to a temperature at which it can be discharged in contact with the atmosphere without combustion or degradation.
While the processes and apparatuses as described in the aforementioned United States patents have been found to provide satisfactory treatment of a variety of raw carbonaceous feed materials to produce an upgraded solid reaction product, there is a continuing need for an apparatus and process which provides for still further efficiency, versatility, simplicity and ease of control in the continuous thermal treatment of a variety of such moist raw carbonaceous feed materials providing thereby still further economies in the conversion and production of high-energy solid fuels as a replacement and alternative to conventional energy sources. The present invention comprises a further improvement in the apparatus and process as described in copending parent application Ser. No. 683,591 including a perforated hearth construction by which hot fluids can be directly injected into the reaction zone in heat transfer relationship with the feed material achieving improved efficiency in heating the feed material with a corresponding increase in the capacity of the apparatus.